LET (linear
energy transfer) is the amount of energy released by a
radioactive
particle or wave over the length of its decay track. Specific
Ionization
is the number of ion pairs produced per unit track length.

High LET radiation
(like alpha & beta particles) ionizes water into H
and
OH radicals over a very short track. In the example, two events
occur in a single cell so as to form a pair of adjacent OH
radicals that recombine to form peroxide, H2O2,
which can produce oxidative damage in the cell.

Low LET radiation
(like X- or gamma rays) also ionizes water molecules, but over
a
much longer track. In the example, two events occur in separate cells,
such
that
adjacent radicals are of the opposite type: the H
and
OH radicals reunite and reform
H2O.